Tobacco is the single biggest cause of cancer in the world

Experts agree that tobacco is the single biggest avoidable cause of cancer in the world 1, 2. Smoking causes over a quarter (28 per cent) of cancer deaths in the UK and nearly one in five cancer cases 3, 4.

And smoking doesn’t only cause cancer. It also causes tens of thousands of deaths each year in the UK from other conditions, including heart and lung problems 3. Worldwide, tobacco caused an estimated 5.1 million deaths in 2004 – that’s one every six seconds 5.

Around half of all regular smokers will be killed by their habit if they continue to smoke 6, 7. And half of those will die in middle age, losing between one and two decades of life compared with non-smokers 1, 3, 7, 8.

Smoking causes many different types of cancer

Smoking increases the risk of at least 14 cancers, including cancer of the lung, larynx (voice box) oesophagus (gullet), mouth and pharynx (upper throat), bladder, pancreas, kidney, liver, stomach, bowel, cervix, ovary, nose and sinus as well as some types of leukaemia 9-11. There is some evidence that smoking could cause breast cancer 11-15.

After lung cancer, the upper aerodigestive tract cancers (mouth, upper throat, voice box and gullet) have the highest proportions of cancer cases caused by smoking – population attributable fractions, or PAFs, of more than six in 10 4.

Smoking greatly increases the risk of lung cancer

More than four in five UK lung cancers are caused by smoking 3, 4. Lung cancer is the most common cause of cancer death in the UK and accounts for around two thirds (68 per cent) of smoking-related cancer deaths 3, 16.

People who smoke were first shown to be more likely to develop lung cancer than non-smokers in 1950 17. This study found that people who smoked around 20 cigarettes a day had 26 times the lung cancer risk of non-smokers. And people who smoked around three cigarettes a day still had six times the lung cancer risk of non-smokers.

After these first results came out, UK scientists began a large long-running study of smoking in British doctors, which Cancer Research UK has helped to fund, that has told us a lot about the dangers of smoking 18. This study has found similarly huge risks associated with smoking. Men who smoked at 25 or more cigarettes a day had over 24 times the risk of dying from lung cancer as people who had never smoked 7.

There are long time lags between changes in the number of people who smoke and the number of people who develop lung cancer due to smoking 1, 5, 8. So rates of lung cancer in the UK reflect smoking rates decades earlier.

Lung cancer risks are highest in people who smoke more cigarettes per day, for more years, from a younger age

We cannot exactly calculate a person’s lung cancer risk based on how many cigarettes they smoke or the number of years they have been a smoker. But research has shown that lung cancer risk is greatest among those who smoke the most cigarettes, over the longest period of time, having started at the youngest age 1, 11.

Although both have an effect on risk, the number of years someone has spent smoking is more important than the number of cigarettes they smoke a day 19. This means that overall, smoking 20 cigarettes a day for 20 years is even worse for you than smoking 40 a day for 10 years.

Even light or occasional smoking is bad for your health

Although the risk of an early death increases the more you smoke, people who think of themselves as light or occasional smokers also have an increased risk compared to people who don’t smoke 20, 21. One study found that people who smoked up to four cigarettes a day were about 50 per cent more likely to die prematurely than non-smokers 20. And the Million Women study found that women who smoked up to 10 cigarettes a day were twice as likely to die prematurely than non-smokers 21.

Large studies looking at the health risks of smoking, such as the British Doctors Study and the Million Women Study, have found that people smoking between 1 and 14 cigarettes a day are at least seven times as likely to die from lung cancer than people who have never smoked 7, 21, 22. And one study found people who smoked between 5 and 9 cigarettes a day had a higher risk of dying from lung cancer, or any type of cancer 20.

Stopping smoking can reduce the risk

A large number of studies have shown that stopping smoking can greatly reduce the risk of smoking-related cancers, compared to continuing to smoke 1, 11. And the earlier you stop, the better 7, 21, 23.

The latest results from the British Doctors’ Study show that stopping smoking before the age of 30 meant that life expectancy remained similar to that of a non-smoker 7,. And even people who quit smoking when they were about 60 years old lost fewer years of life to smoking than those who continued 7,.

Cutting down is a good start, but the big benefits come from quitting

The evidence for the health benefits of cutting down the number of cigarettes you smoke is mixed. Some studies have shown that reducing your cigarette consumption, compared to carrying on smoking the same amount, has benefits for lung cancer risk and overall survival 24-26. But other studies found benefits only with quitting completely 27, 28.

However if you reduce the amount you smoke you may be more likely to stop smoking in future 29-33. So although you only experience the full health benefits if you stop smoking altogether, cutting down can be a good first step, if you find it too difficult to quit completely in one go.

Tobacco smoke contains many dangerous chemicals

Scientists have identified over 5,300 different chemicals in tobacco smoke 11. The International Agency for Research into Cancer (IARC), the gold standard for establishing the causes of cancer, state that there are more than 70 chemicals in tobacco smoke that have been found to cause cancer in studies involving people or in the laboratory 11. And many of the other thousands of chemicals are toxic and harmful to your health, including carbon monoxide, hydrogen cyanide and ammonia 1, 34.

In 1994 tobacco companies in the US released a list of 599 different cigarette additives, which included chocolate, vanilla, sugar and liquorice as well as common herbs and spices 35. Although these don’t make cigarettes any more toxic, they are meant to make cigarettes taste nicer and ensure that smokers want to continue smoking.

Flavoured tobacco, such as menthol or chocolate-flavoured cigarettes, will be banned in Europe from 2016 under the Tobacco Products Directive 36. Although additives necessary for manufacturing tobacco can continue to be used, including sugar lost during the curing process, tobacco companies will have to do more research into additives with the possibility that those found to be more harmful or addictive could be banned from use 36.

Chemicals in tobacco smoke can damage the body in many ways

Chemicals found in tobacco smoke can damage DNA 1, 11, 37. For example, studies have shown that benzo(a)pyrene damages a gene called p53 that normally protects our cells from cancer 37, 38. While polonium-210 becomes concentrated in hotspots in smokers’ airways, subjecting them to very high doses of high-energy alpha-radiation that damages the DNA of nearby cells 39.

The cocktail of chemicals in tobacco smoke is even more dangerous as a mix. Toxic metals found in tobacco smoke, like cadmium, arsenic, and lead, stop our cells from repairing DNA damage 40. This worsens the effects of chemicals like benzo(a)pyrene that damage DNA and makes it even more likely that damaged cells will eventually turn cancerous.

Many tobacco poisons disable the cleaning system that our bodies use to remove toxins. Some substances including formaldehyde and acrolein kill cilia, tiny hairs in our airways that help to clear away toxins 37.

Nicotine is a very addictive drug

Most smokers do not smoke out of choice, but because they are addicted to nicotine 37, 41. This was highlighted in a report by The Royal College of Physicians into the effects of nicotine 41. They also compared nicotine to other supposedly ‘harder’ drugs such as heroin and cocaine. They looked at many things including how these drugs cause addiction, how difficult it is to stop using them and how many deaths they caused. The report concluded that nicotine is a highly addictive substance, particularly when people are exposed to it through using tobacco, and that tobacco dependence is at least as serious as addiction to ‘harder’ drugs 41.

People associate smoking with feeling less stressed and anxious, but the evidence suggests this is only because it temporarily relieves the unpleasant symptoms of nicotine withdrawal 37, 41. If anything, current smokers seem to feel more stressed and anxious than ex-smokers or people who have never smoked 41.

Smokers can also make mental links with abstract things like the smell of cigarettes, objects related to smoking like ashtrays and lighters, and situations in which they usually smoke. These can all act to reinforce the addiction to smoking 37.

Smokers are still exposed to dangerous chemicals if they smoke filtered or ‘low-tar’ cigarettes

Current smokers have a higher risk of lung cancer than former smokers or never smokers, whatever type of cigarettes they smoke 1, 11. There is no difference between the risk of dying from lung cancer for smokers of cigarettes ‘rated’ as medium, low and very low in tar 42. But smoking unfiltered high tar varieties makes the risk even higher 1, 11, 42.

The amounts of tar or nicotine associated with a particular variety of cigarette are based on machine measurements of smoking. But the measurements don’t accurately reflect the amounts actually received by a smoker 1, 34, 37, 43. Research that measured markers of nicotine and certain cancer-causing chemicals in smokers found that they took in the same amounts whatever the tar rating of the cigarettes they smoked 44. Smokers change the way they smoke – such as taking bigger or more frequent puffs, blocking filter holes or smoking more cigarettes in total – in order to satisfy their nicotine cravings 1, 11, 37, 43.

In 2003 EU legislation came into force banning tobacco companies from using phrases like ‘light’, ‘mild’ or ‘low-tar’ to describe their products because these could mislead people into believing such products were safer or healthier 45.

Alcohol worsens the effect of smoking

Alcohol has also been shown to be a cause of mouth, oesophageal and liver cancers, among others 11. And scientists have found that together, the effects of alcohol and tobacco are worse than for either one of them by itself 11, 46-49.

Passive smoking causes cancer and other diseases

There is clear evidence that breathing in other people’s smoke causes cancer in non-smokers 1, 11, 37,50. Second-hand smoke, also known as environmental tobacco smoke or passive smoking, exposes people to cancer-causing chemicals 11.

People who have never smoked have their risk of lung cancer increased by around a quarter if they have colleagues who smoke at work or have a spouse who smokes 50-52. The risk increases the more second-hand smoke they’re exposed to, workers exposed to the highest levels can have their risk of lung cancer doubled 51, 52.

Second-hand smoke can reach high levels in enclosed spaces such as within the home or inside a car 53-57. Studies have shown that even when you open the windows levels can be dangerously high 53, 55, 57, 58.

Second-hand smoke also causes other health problems in non-smokers including heart disease and respiratory diseases including asthma in children 37, 50, 59. And it may increase the risk of pharyngeal and laryngeal cancers 10, 11.

Tobacco in pipes, cigars, bidis and shisha can also cause cancer

It’s not just cigarettes which increase the risk of cancer – other ways of smoking tobacco including pipes, cigars, bidis and shisha are also linked with the disease 1, 11, 60, 61. Reviews of the evidence by the International Agency for Research on Cancer have concluded that smoking cigars, pipes or bidis increases the risk of lung, mouth, throat, voice box, gullet (oesophagus) and stomach cancers 1, 11. And cigar smoking has also been linked with pancreatic cancer 1, , 60, 62. Shisha smoking has been less well studied but the evidence suggests it is linked to lung cancer and possibly other cancers 63, 64.

Because they think it is healthier, some cigarette smokers switch to smoking a cigar or pipe. But aside from the health risks associated with pipes and cigars, research has also shown that because these people are more likely to inhale the smoke than people who have only ever smoked a cigar or pipe, the switch makes little difference to their chance of dying as a result of smoking 60.

Smokeless tobacco can also cause cancer

There are a wide variety of smokeless tobacco products used around the world 11, 65. In the UK chewing tobacco, which may be used as part of betel quid, paan or zarda, is popular with South Asian communities. Tobacco can also be chewed or sucked by itself, or taken as snuff.

Smokeless tobacco has been shown to cause mouth, gullet (oesophageal) and pancreatic cancers 11,, 66. Chewing betel quid with tobacco has also been shown to cause cancer of the mouth, upper throat and gullet (oesophagus) 11,. The size of the cancer risks associated with using smokeless tobacco are likely to vary depending on the type of product and the way it’s used 65.

It has been suggested that smokeless tobacco could be used to help people stop using smoking tobacco, and that falling smoking rates in Sweden have been linked to people switching to a type of smokeless tobacco called ‘snus’. But a review of the evidence by the IARC in 2007 concluded that the available evidence does not support these claims 66.

There are many cancer-causing chemicals present in smokeless tobacco, including in ‘snus’ 11, 65, 66,. Studies have shown that these harmful chemicals, such as tobacco-specific nitrosamines (TSNAs), are taken up by people using these products 65.

E-cigarettes have not yet been proven to support smoking cessation

Although some users have claimed that e-cigarettes have helped them stop smoking, there is not enough scientific evidence to show whether they are effective, and as safe as current licensed cessation aids (such as nicotine patches) 67-72. So far there has only been one trial comparing the use of e-cigarettes to another stop smoking aid (patches in this case) 73. Unfortunately this trial was too small to show a significant difference between the likelihood of quitting 73.

However e-cigarettes are almost certainly far safer than smoking tobacco 74, 75. E-cigarettes may play a role in helping smokers quit but larger trials are needed as scientific evidence that they really work, to explore how best to use them and to see how they compare to other methods of quitting 72.

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